Why would my PO come by my apartment? HELP!

Okay, first off thanks for reading this

I started probation on August 27th for the marijuana possession ticket I got back in January. My first test was last friday, September 24th. I had smoked only once 2 weeks before my test, and then a week before that (so 2 times total ahah) and I drank what I thought was a good amount of water and I took a detox kit the morning of (its the kind that will only work for a few hours). I am 20 years old and a girl, and I'm an average size so I didn't think I would have a high level of THC, especially with the detox.
Anyways, I was at class tonight and apparently my probation officer stopped by my apartment to see if I was home and if he could talk to me, but obviously I wasn't. My roommate told me I'm supposed to call him tomorrow morning.
Has anyone who's been on probation had this happen to them? Is this normal protocol to inform someone they've failed their test? (I assumed that if I failed he would tell me at our next meeting on October 18th). Also, he's already been by the apartment once before to verify that I live there.
I'm really worried because my parents don' t know about any of this and although I'm 20, they're still paying for college and my apartment. I definitely still have to answer to them.
Any help would be beyond greatly appreciated! I'm a natural worrier, and I can't think of a damn reason to come by the place. Also, I've never had experience with the law before so I'm quite a n00b at this hahah.

Thanks guysss!